Transplanted tissue is attacked by T lymphocytes and antibodies; both recognize foreign (allogenic) HLA major histocompatibility complex (MHC) antigens. In order to better understand the immunopathology of transplant rejection, our long-term goal is to study antigenic sites (epitopes) on the human HLA-B7 transplantation antigen. Specific Aim 1: T Lymphocyte Epitopes. Two competing hypotheses describe how T lymphocytes recognize HLA allogenic epitopes. In the "MHC only" hypothesis, T lymphocytes recognize HLA antigens alone, regardless of bound peptide; in the "self peptide" hypothesis, T lymphocytes recognize an array of cellular peptides that are bound to allogenic HLA molecules. To judge these hypotheses we will test a panel of anti-HLA-B7 T lymphocyte clones from four donors against a series of 31 HLA-B7 variants. Variants with mutations in HLA surface amino acid residues will test the "MHC only" hypothesis, whereas variants with mutants in HLA peptide- binding groove amino acid residues will test the "self peptide" hypothesis. Specific Aim 2: Antibody Epitopes. We will test whether HLA-B7 epitopes fit the paradigm of soluble protein antigens, in which antibody epitopes are large, depend on three-dimensional conformation, include both variable and invariant amino acid residues, and are rarely affected by mutations at distant sites. To judge this hypothesis we will study four monoclonal antibodies (mAbs) that bind to physically non-overlapping sites on HLA-B7. We will test 15-20 HLA-B7 variants for each of the epitopes under study; each variant will have a point mutation within 15 A of a site known to control antibody binding. The pattern of antibody binding to these variants will allow us to assess whether HLA epitopes, like the soluble protein paradigm, are large and conformational, include both variant and invariant amino acid residues, and are spatially discrete. Health Relatedness. In addition to transplant rejection, HLA molecules also regulate the immunopathology of autoimmune disease and cancer. Thus, if we are to understand transplant rejection, autoimmune disease, and cancer, we must resolve how T lymphocytes and antibodies recognize HLA transplantation antigens. Such an understanding may lead to more specific treatment of these conditions.